Radio frequency coupling device



April 30, 1957 o. 0. FIET ETAL RADIO FREQUENCY COUPLING DEVICE OriginalFiled March 28, 1952 /A/|//Y7'0E5J OWEN E3. FIET EI-IHRLES Pu 1.x

BY M 5 AW ATTORNEY United States Patent RADIO FREQUENCY COUPLING DEVICEOwen 0. Fiet, Oaklyn, and Charles Polk, Westmont, N. L,

assignors to Radio Corporation of America, a corporation of DelawareContinuation of application Serial No. 279,138, March 28, 1952. Thisapplication October 7, 1955, Serial No. 539,192

9 Claims. (Cl. 333-313) This invention pertains to a device for couplingradio frequency energy from a transmission line to a radio frequencyradiating element, and particularly to a tuned coupling deviceutilizable to extract the energy from a transmission line and apply thesame to an antenna. This application is a continuation of application,Serial No. 279,138, filed March 28, 1952, now abandoned.

When a radiating element, such as an antenna, is coupled to atransmission line, it is desirable to have the radiating element appearas a pure resistance to the transmission line. Such a condition isespecially desirable for an array of radiating elements fed from acommon line. Even if the radiating element itself is resonant andappears purely resistive to the transmission line, capacitive orinductive coupling of the radiating element to the line will introduce areactance component which may adversely effect the impedance of thearray as seen from the feedpoint. Under other circumstances, it may bedesirable for optimum gain of an antenna system to use a type ofradiating .element whose impedance has a reactance component at theoperating frequency.

The present invention is directed to a radio frequency coupling devicein which ,a reactance element is introduced into the coupling elementitself to tune the combination of the radiating element and the couplingdevice to resonance and cause the combination of radiating element andcoupling device to appear as a resistive load to the transmission line.

On the other hand, to fulfill certain conditions of current magnitudeand phase in an array of radiating elements, the coupling loop of thisinvention may be adjusted to provide a complex impedance as seen fromthe transmission line to thereby obtain the required current conditions.

It is an object of this invention to provide an improved radio frequencycoupling device capable of tuning out the eifective reactance of aradiating element as seen from the transmission line to which it iscoupled.

It is another object of this invention to provide a radio frequencycoupling element for coupling a load device to a transmission linewherein the coupling element contains as a part thereof a reactancearranged to tune out the reactance of the load device at a desiredoperating frequency.

A further object of this invention is to provide a radio frequencycoupling element for coupling a load device to a transmission linewherein the coupling element contains as a part thereof a reactance ofadjustable value to present a complex impedance of desired magnitude andphase to the transmission line to which it is coupled.

Briefly, in accordance with this invention, there is provided a radiofrequency coupling device for coupling a load, as for example aradiating element, to a transmission line, wherein the coupling deviceitself contains a lumped reactance which may be of the same or ofopposite sign to the reactance presented by the radiating element. Inone application of the invention, thc coupling 2,790,958 I Patented Apr.30, 1957 device may be tuned so that the entire coupling device andradiating element system appear as a pure resistance as seen from thetransmission line. For another application, the coupling device may beused to make the entire coupling device and radiating element systemappear as a complex impedance of a desired magnitude and phase to thetransmission line to which it is coupled.

In one embodiment, the coupling device is a generally.

U-shaped loop extending within the enclosing side wall of a transmissionline, such as a coaxial transmission line or a waveguide. The loopincludes a discontinuous conductive rod with the adjacent ends of therod at the discontinuity supported in insulated capacitive relationshipby means of a dielectric sleeve surrounded by a conductive sleeve. Theterminal ends of the conductive rod constitute the terminals of the loopwhich are connected to the radiating element. When the reactance of theradiating element and the coupling device without the capacitive sleeveappear inductive, the entire coupling device (including capacitivesleeve) and radiating element together may be made to appear as a tunedcircuit at a desired resonant frequency. The combination of couplingdevice and radiating element therefore acts as a purely resistive loadto the transmission line to which it is coupled.

According to another form of the invention, the con- 1 pling loopcomprises a continuous U-shaped conductive rod having ends surrounded bydielectric sleeves which are in turn surrounded by conductive sleeves.The conductive sleeves may be considered to be a single discontinuoussleeve having terminal ends constituting the ter minals of the loop.

A more detailed description follows in connection with a drawing,wherein:

Figure 1 illustrates one arrangement of the coupling device of thepresent invention;

Figure 2 illustrates a modification of the coupling device of thisinvention;

Figure 3 is an elevation and Figure 4 is an end view, partly in section,of an application of one form of the coupling device of this inventionutilized to feed a slot radiator, and

Figure 5 is another modification of the coupling device of thisinvention.

Referring now to Figure 1, there is shown a radio frequency couplingdevice according to the present invention in association with a coaxialtransmission line having an inner conductor 11 and an outer conductor13. The coupling device of this invention consists of a generallyU-shaped loop having a discontinuous metallic loop member or rod in theform of two L-shaped conductive members 15 and 17 maintained ininsulated spaced relationship by a sleeve of insulation 19, which sleeveof insulation is surrounded over a portion of its length by a. metallicsleeve 21. The insulation sleeve 19 is provided with two spaced holes atopposite ends and in the same straight line into which the adjacent endsof the L-shaped members 15 and 17 are inserted for the desired distance.The construction provides a capacitor in series with the loop. A load,illustrative of a radiating antenna element and shown as composed of aresistance 23 and an inductive reactance 25, is shown electricallyconnected across the terminal ends of the discontinuous conductive rodwhich constitute the terminal ends of the loop. The inductive reactance25 may be assumed to be the resultant reactance of the loop and theradiating element, considered together.

of the load together with the inductive reactance of thev loop to form aresonant circuit.

anonons metallic sleeve 22', and at the other end to the second:

metallic sleeve 22'. The loaddevice, shown as composed of a: resistance23 and an inductive reactance 25,

like the same load device in Figure i is then connected between themetallic end sleeves 22 and 22'. The conplingt loop. may be viewed ascomprising a continuousconductive rod 16 surrounded by a discontinuousdielectric sleeve 20', 20" and a discontinuous conductive sleeve 22, 22;ductive sleeve constitute the terminals of the loop.

Referring: now to Figures 3 and' 4'; Figure 3 shows anelevation: andFigure 4 shows anend view, partly in section, 033 an application of theradio: frequency couplingdevice of this invention to feed a.slottedcylinder antcnnat. The transmission line in this case has aninner conductor: 11' and an outer cylindrical wall conductor 13 having aplurality of slots 27 cut through the outer cylindrical wall conductor13. The radio frequency coupling device is like that shown in Figure l,and the same numerals areused to identify'the component parts thereof.and 17 support the sl'eeveofi insulation 19 which is surrounded over aportion of its lengthby the metallic The terminal ends 05' the rods Hand17- are' sleeve 21?. conductivelyr connected by metal brackets 29 toopposite sides of the slot 27 near the electrical center thereofl As. isevident from Figure 4; three such slots 27 ina single layer may be fedby in-phase voltages fromhiifererent. coupling devices respectivelyacross the slots.

The voltage fed to any slot 27 may be reversed 180* in phase by thecouplingdevice of the present invention by reversing the connection ofthe loop to-the opposite sidesof the slot 27.

The tuned coupling device should penetrate into the space between theouter cylindrical wall 13" and the inner cylindrical conductor 11 from0.15 to 0.3 of' the distance between the inside face of the outercylindrical wall 13 to the outer surface of the inner conductor 11-.More than30 percent penetration (03* oftheinterwall'distance) is notpreferredbecause the couplingloop thenofl'ers an objectionable amount oflumped shunt capacity acrossthe' transmission line 1 1', 13". Less thanpercent penetration, on-the' other hand, does notprovidc sufiicientcoupling with the magnetic fieldinside the transmission line to developthe optimum driving voltage across the feedpoint of the slots 27-'inthe-outer cylindrical wall 1'3l The penetration of the coupling deviceof this invention determines the degree of coupling of the tuned loop tothe transmission line 11-, 13'. To provide a proper impedance'matchbetween the transmission. line and the antenna elements being coupledthereto, the degree of penetration of the inter-wall distance by thecoupling loop will depend'upon the following thrceparametcrs: thecharacteristicimpedance of the transmission line, the impedance oftheradiating elements at the frequencyfor which optimum coupling isdesired; andthe-numberof' antenna elements to be coupledi Thepositioningof the metal brackets 29 and'rnethod of connecting the radiofrequency coupling device is shown with the axis of themetal sleeve 21.and thebottom of the loop parallelto. the.v axis of the. transmissionline 11',.13'. As. will be understood, the voltage induced across theslots by the transverse-magnetic field: within the transmissionline 111,1-3 in-Figurest 3tand 4. is proportional to the cosine of theangle.between the.

Each of the The terminal ends of the discontinuous con- The adjacentends of the two- L-shaped rods-15 4 axis. of. the sleeve 21. of. the.coupling. loop. and the axis of the transmission line 11',.13.

An adjustment of the coupling loop of the present invention so that theaxis of the sleeve 21 is not parallel to the axis of the transmission.line-11, 13' may thus serve to adjust the. magnitude of the currentinduced in the loop. The phase of the current in the loop may beadjusted by changing the length: of the sleeve 21 which is in capacitiverelationship tothe split loop members 15 and 17. This is preferably doneby changing the length of the metallic sleeve 21 but may also beaccomplished by changing the penetration of either or both of the rods15 and 17 within the insulating sleeve 19.

It should be noted at this point that although three slots are shown inFigure 4 in theperiphery of the outer cylindrical wall 13, a layer ofslots in a slotted cylinder antenna may contain: one or more slots up toa number commensurate with. the structural strength requirements of thecylinder itself; Although. forming no part of this invention per se; itis pointed out that the: angular po sition of the layer ofi slots next.adjacent to those illustrated in Figure 4, abovevor-below, may berotated to an intermediate; position to provide improved circularity ofthe radiatedtfield. AISOgIhB slots27 in a; slotted cylinder antenna maybe located: around only part of the periphery of the outer cylindricalwall. conductor 13. With any of these alternatives, the tuned couplingdevice of this invention is particularly adapted to tuning out thecombination of thereactance of the slots 27 and the coupling devices; sothat the entire combination of coupling devices and radiating elementspresent a resistive load: to the transmission line 11', 13.

As an illustration. of; how the feature of reversing the phase of thevoltage applied across the slots of a cylindrical slot antenna isutilized in practice, consider the case of two adjacent layers of slotsspaced apart along the length of the. cylinder by an odd multiple. ofone-half wavelength. Since the magnetic field inside the trans missionline will be reversed 180 electrical degrees away from any couplingpoint (that is,.tWo points spaced an odd multiple of half-wavelengthsapart will be 180 out of phase), it the. coupling loop: in one layer ofslots has its terminals reversely' connected. to its slot relative to.the terminals of: the coupling loop in the slot in the next layer alongthe length of the cylinder, the voltage applied across these twoslots bytheir coupling loops will be in. phase.

lnFigure S, there is-shownzamodification of the radio frequencycoupling. device of the present invention in association with. a radiofrequency feed? source having an enclosing side wall 14. According tothis embodiment, the coupling device consistsof a U-shaped loop member18 extending through the enclosing side wall 14. Intermediate the lengthof the loop 18 is an inductive metallic sleeve 31. Coaxial with thestraight portion of the loop 18inside the enclosing side wall 14, theinductive metallic sleeve 31 is conductively connected to the U-shapedloop 18at one end of the metallic sleeve 31 and intermediate thelengthlof the sleeve 31,.

The arrangement shown in Figure 5 is especially adapted to tune out thereactance of a load device such as an antenna which is capacitive incharacter. In this instance, the load is represented as consisting of aresistance 23 and a. capacitive reactance. 26, electrically connectedacross the ends of the metallic loop 18.

In accordance with one embodiment ofthe present invention, the radiofrequency coupling device was utilized to feed a slotted cylinderantenna having eighteen layers of slots, each layer consisting of threeslots each symmetrically spaced apart around the cylinder. The slottedcylinder was of galvanized steel tubing 6% inches in diameter and34 feetlong. The individiialslots were 1 inch wide and approximately 13wavelengths long parallel to the axis ofthecylinder.

Adjacent layers of slots were staggered orrotated60' relative to eachother to obtain maximum mechanical strength and a circular horizontalpattern. Radio frequency energy in the band from 842 megacycles to 848megacycles was distributed to the eighteen layers of slots by means of asingle coaxial line feeder system having a 1% inch diameter copper tubeas the inner conductor, and the slotted steel cylinder as the outerconductor. Each of the slots was 18.1 inches long. The center of eachlayer was spaced 1.5 wavelengths, 20.9 inches at this frequency, fromthe center of the next adjacent layer. In the above-described slottedcylinder antenna, the combination of 1.3 wavelength slots and eighteenlayers of such slots was arrived at to give a high gain with the leastnumber of layers in a given total antenna aperture. A slot length of 1.3presents an antenna impedance of each layer which is slightly inductivein character, allowing the slot to be adjusted to present a resistiveload with the capacitive coupling loop of Figures 1, 3 and 4 of thisinvention. Theoretically, it is possible to extend the length of such aslot to 1.4x or 1.45x, but the impedance of slots of that length is ofsuch value that it becomes more difiicult to tune the slots to appear asa resistive load to the transmission line. Further, with thisconstruction, where the slots of one layer do not overlap along thelength of the cylinder with another layer, a stronger structure results.

The tuned coupling devices of this invention were utilized to feed allof the slots in phase with equal voltages to provide a purely resistiveload to the common transmission line. Utilizing a bracket 29 like thatshown in Figures 3 and 4 to suspend the coupling loop of Figares l, 3and 4 into the space between the inner and outer walls of thecylindrical antenna, the coupling loop of this invention had thefollowing dimensions: The L- shaped members and 17 were solid cylinderaluminum having an outside diameter of /s inch, and the total loop was2% inches long measured between the ends of the upstanding portions ofthe L-shaped members 15, 17. The insulating sleeve 19 was of apolytetrafluoroethylene material marketed under the trade name Teflonand had an outside diameter of inch and a length of 1 inches, and fittedtightly around the rods 15 and 17 to maintain an end-to-end spacing of Ainch therebetween. The metal sleeve 21 was of copper and was pressfitted on the Teflon insulating sleeve 19 and had an outside diameter of%4 inch and a length of 0.875 inch. The coupling loop was set topenetrate the inter-wall distance between the outside steel tube 13' andthe inner copper cylinder 11 a distance of .42 inch measured from theinside of the outer wall 13' to the center of that portion of the rods15 and 17 parallel to the axis of the cylinder.

Although the capacitance of the coupling device just described could bechanged either by changing the length of the metallic sleeve 21 or byadjusting the penetration of the rods 15 and 17 into the insulatingsleeve 19, it was found more convenient in practice to adjust the tuningof the loop by cutting down the length of the metallic sleeve 21 untilthe proper capacitance was obtained to tune the loop and slot toresonance.

As has been described in detail above, the radio frequency couplingdevice of this invention is capable of tuning out the effectivereactance of a load device (such as a slot antenna) at a desiredfrequency of resonance. Although the invention has been explained inconnection with an arrangement for coupling a radiating system to atransmission line, it should be understood that the present invention isequally valuable for intercoupling one or a plurality of generators to asingle load device, which would be the case, for example, where aplurality of receiving antennas are being coupled to a commontransmission line.

Further, although the operation of the coupling device has been set outwith a particular view to explaining the condition where the couplingdevice is utilized to tune the load to resonance, it is equally valuablefor obtaining a desired reactance component for the load device tofulfill certain required conditions of current magnitude and phase for aplurality of such load devices. The coupling loop of this invention isparticularly uti-.

lizable to extract energy from a transmission line to apply that energyto an antenna. The loop portion couples to any magnetic field within atransmission line having an enclosing side wall, such as a coaxial lineor a waveguide. The plane of the coupling loop itself, of course, mustnot be parallel with the direction of the magnetic field to whichcoupling is desired, since the voltage induced in the loop isproportional to the sine of the angle between the plane of the loop andthe direction of the magnetic field. Maximum coupling therefore will beobtained when the plane of the loop is precisely trans verse to thedesired magnetic field.

What is claimed is:

l. A radio frequency coupling device for coupling energy from atransmission line having an enclosing side wall to a radiating element,comprising a discontinuous metallic loop member extending within saidside wall and adapted to have the ends thereof conductively connected tosaid radiating element, and a lump reactance intermediate the ends ofsaid loop member and within said side wall, said lumped reactanceincluding a portion of said loop, an insulating sleeve surrounding atleast the discontinuous portion of said loop, and a metallic sleevepositioned around said insulating sleeve and in capacitive relation tosaid discontinuous loop.

2. A radio frequency coupling device for coupling energy from atransmission line having an enclosing side wall to a radiating element,comprising a generally U- shaped coupling loop extending within saidside wall and having terminal ends adapted for conductive connection tosaid radiating element, said loop including a conductive rod element, adielectric sleeve surrounding at least a portion of said rod element,and a conductive sleeve element surrounding at least :a portion of saiddielectric sleeve, one of said conductive elements being discontinuousand having remote ends constituting said terminal ends of said loop,said conductive rod element, said dielectric sleeve, and said conductivesleeve element cooperating to present a series capacitance in said loop.

3. A radio frequency coupling device as defined in claim 2 wherein saidconductive rod is discontinuous and has remote ends constituting theterminal ends of said loop.

4. A radio frequency coupling device as defined in claim 2 wherein saidconductive sleeve is discontinuous and has remote ends constituting theterminal ends of said loop.

5. A radio frequency coupling device for coupling energy from atransmission line having :an enclosing side wall to a radiating element,comprising a coupling loop extending within said side wall and havingterminal ends adapted for conductive connection to said radiatingelement, said loop including a continuous generally U-shaped conductiverod element having a center portion and two end portions, two dielectricsleeves each surrounding a respective one of said end portions, and twoconductive sleeve elements each surrounding a respective one of saiddielectric sleeves, said conductive sleeves constituting the terminalends of said coupling loop, whereby two capacitors are provided inseries with said loop.

6. A radio frequency coupling device for coupling energy from :atransmission line having an enclosing sidewall to a radiating element,comprising a generally U- shaped conductive coupling loop extendingwithin said side wall, said coupling loop being discontinuous andincluding a conductive rod, a dielectric sleeve surrounding a portion ofsaid rod, :and a metallic sleeve surrounding said dielectric sleeve,said conductive rod, said dielectric sleeve, and said metallic sleevecooperating to present a series capacitive reactance in said couplingloop.

7. A radio frequency coupling device for coupling energy from atransmission line having an enclosing side Wall to a radiating, element,comprising a generally U-shaped coupling loop extending within saidsidewall,

said loop including metallic conductor means, dielectric meanssurrounding at least a portion of said conductor means, and metallicsleeve means surrounding at least a portion of said dielectric means,one of said metallic means being discontinuous and constituting theterminal ends of said loop.

8. A radio frequency coupling device for coupling energy from atransmission line having an enclosing sidewall to a radiating element,comprising a balanced coupling loop extending within said sidewall, saidloop including metallic conductor means, dielectric means surrounding atleast a portion of said' conductor means, and metallic sleeve meanssurrounding at least a portion of said dielectric means, said metallicconductor meansbeing discontinuous and constituting the terminal ends ofsaid loop.

References Cited in the file of this patent UNITED STATES PATENTS2,395,165 Collard Feb. 19, 1946 2,523,254 Talpey Septv 19, 19502,543,085 Willoughby Feb. 27, 1951 2 ,609,450 Early Sept. 2, 1952

